In an electrophotographic image forming apparatus, an image is generally formed on a surface of a sheet such as a paper sheet or a plastic film through the following process steps.
First, a surface of a photoreceptor body is evenly electrically charged and, in this state, exposed to light, whereby an electrostatic latent image corresponding to an image to be formed on the sheet is formed on the surface of the photoreceptor body (charging step and exposing step).
In turn, toner (minute color particles) preliminarily electrically charged at a predetermined potential is brought into contact with the surface of the photoreceptor body. Thus, the toner selectively adheres to the surface of the photoreceptor body according to the potential pattern of the electrostatic latent image, whereby the electrostatic latent image is developed into a toner image (developing step).
Subsequently, the toner image formed by the development is transferred onto the surface of the sheet (transfer step), and fixed to the surface of the sheet (fixing step). Thus, the image is formed on the surface of the sheet.
Further, toner remaining on the surface of the photoreceptor body after the transfer of the toner image is removed by a cleaning blade or the like kept in abutment against the surface of the photoreceptor body (cleaning step). Thus, the photoreceptor body is ready for the next image formation.
In the developing step out of the aforementioned process steps, a developing roller is used for developing the electrostatic latent image formed on the surface of the photoreceptor body into the toner image.
The developing roller typically includes a roller body produced, for example, by forming an electrically conductive rubber composition into a tubular body and crosslinking the rubber composition of the tubular body.
An ion conductive rubber such as epichlorohydrin rubber is used for a rubber component of the rubber composition in order to impart the roller body with electrical conductivity.
Further, a diene rubber such as acrylonitrile butadiene rubber (NBR) is used in combination with the ion conductive rubber for the rubber component in order to impart the roller body with rubber characteristic properties, i.e., in order to make the roller body flexible and less susceptible to permanent compressive deformation with a smaller compression set.
The outer peripheral surface of the roller body is generally covered with a coating film to be thereby properly conditioned.
The coating film is generally formed by applying a liquid coating agent onto the outer peripheral surface of the roller body by a spraying method, a dipping method or the like and then drying the applied coating agent. Therefore, the coating film is liable to suffer from contamination with dust and other foreign matter during the formation thereof, thickness unevenness and other defects.
For preparation of the coating agent, an organic solvent is required. The use of the organic solvent may exert a great load on the environment, and go against a recent trend toward reduction of VOC (volatile organic compounds).
An oxide film may be formed in the outer peripheral surface of the roller body by irradiation with ultraviolet radiation, thereby obviating the coating film (Patent Document 1).
Where the roller body is produced by using the aforementioned conventional combination of rubbers for the rubber component and has an outer peripheral surface formed with a thin oxide film, the roller body has a lower wear resistance and, therefore, is liable to be significantly worn as the number of times of image formation increases.
Due to wear debris generated by the wear of the roller body, the cleaning blade in abutment against the photoreceptor body is liable to be torn up or reversed as the photoreceptor body is rotated, making it impossible to properly clean the surface of the photoreceptor body. Problematically, this may result in formation of defective images.